Question

在mongo中，在使用aggregation执行$lookup之后，我希望请求只返回一些字段，而不是整个文档。

我有以下查询：

db.somecollection.aggregate([{
    $lookup: {
        from: "campaigns",
        localField: "campId",
        foreignField: "_id",
        as: "campaign"
    }
}, {
    $unwind: "$campaign"
}, {
    $lookup: {
        from: "entities",
        localField: "campaign.clientid",
        foreignField: "_id",
        as: "campaign.client"
    }
}]);

此请求将返回给我：

{
"_id" : ObjectId("56cc7cd1cc2cf62803ebfdc7"),
"campId" : ObjectId("56c740e4479f46e402efda84"),
"articleId" : ObjectId("56c742c06094640103ba3843"),
"campaign" : {
    "_id" : ObjectId("56c740e4479f46e402efda84"),
    "clientid" : ObjectId("56c740b8479f46e402efda83"),
    "client" : [
        {
            "_id" : ObjectId("56c740b8479f46e402efda83"),
            "username" : "someusername",
            "shhh" : "somehashedpassword",
            "email" : "mail@mail.com",
        }
    ]
}

该请求效果很好，但我想过滤campaign.client中的字段，仅获取_id和username。有没有办法在MongoDB aggregate请求中执行此操作？

Answer 1

为了帮助其他人，@ SiddhartAjmera有正确的答案，我只需要为“campaign.clientid”这样的嵌套值添加双引号。

最终代码应为：

db.somecollection.aggregate([
      {
        "$lookup": {
          "from": "campaigns",
          "localField": "campId",
          "foreignField": "_id",
          "as": "campaign"
        }
      },
      {
        "$unwind": "$campaign"
      },
      {
        "$lookup": {
          "from": "entities",
          "localField": "campaign.clientid",
          "foreignField": "_id",
          "as": "campaign.client"
        }
      },
      {
        "$project": {
          "_id": 1,
          "campId": 1,
          "articleId": 1,
          "campaign._id": 1,
          "campaign.clientid": 1,
          "campaign.client._id": 1,
          "campaign.client.username": 1
        }
      }
]);

Answer 2

在pipeline内使用$project和$lookup

db.somecollection.aggregate([{
    $lookup: {
        from: "campaigns",
        localField: "campId",
        foreignField: "_id",
        as: "campaign"
    }
}, {
    $unwind: "$campaign"
}, {
    $lookup: {
        from: "entities",
        let: { client_id: "$campaign.clientid" },    
        pipeline : [
            { $match: { $expr: { $eq: [ "$_id", "$$client_id" ] } }, },
            { $project : { _id:1, username:1 } }
        ],
        as: "campaign.client"
    }
}]);

Answer 3

只需在前面的答案中添加一些内容即可：您可以在要忽略的项目项上添加0，其余部分将被检索，因此您无需使用1：编写所有列表。 / p>

db.somecollection.aggregate([
  {
    "$lookup": {
      "from": "campaigns",
      "localField": "campId",
      "foreignField": "_id",
      "as": "campaign"
    }
  },
  {
    "$unwind": "$campaign"
  },
  {
    "$lookup": {
      "from": "entities",
      "localField": "campaign.clientid",
      "foreignField": "_id",
      "as": "campaign.client"
    }
  },
  {
    "$project": {
      "campaign.client.shhh": 0
    }
  }
])

Answer 4

我知道现在回答这个问题太晚了。但我认为，有时更新可能会非常有益。
项目阶段很棒，但是在$lookup阶段，您仍然需要整个坞站。仅在紧随其后的投影阶段过滤字段。

MongoDB 3.6发布后，您现在可以将管道添加到$lookup阶段，以指定多个联接条件。在他们的官方文档中找到更多详细信息。
Specify Multiple Join Conditions with $lookup

您可以按以下方式转换聚合管道，以获得所需的结果：

db.somecollection.aggregate([{
    $lookup: {
        from: "campaigns",
        localField: "campId",
        foreignField: "_id",
        as: "campaign"
    }
}, {
    $unwind: "$campaign"
}, {
    $lookup: {
        from: "entities",
        let: {clientid: '$campaign.clientid'},
        pipeline: [
           { '$match': 
             { '$expr': 
               { 
                  '$eq': ['$_id', '$$clientid']   
               }
             }
           },
           { '$project': 
              '_id': 1,
              'username': 1
           }
        ]
        as: "campaign.client"
    }
}]);

这样，您可以在$lookup阶段内过滤联接集合的字段。
注意内部管道的$$阶段内的$match符号。它用于表示在let块中定义的自定义字段。

使用$ lookup进行MongoDB聚合会限制某些字段从查询返回

4 个答案: